<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport"
        content="width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        ul {
            display: flex;
            flex-wrap: wrap;
            justify-content: space-around;
            align-content: space-around;
            width: 650px;
            height: 500px;
            margin: 100px auto;
            background-color: #cccccc;
        }

        ul li {
            width: 200px;
            height: 200px;
            background-color: #999999;
            list-style: none;
        }
    </style>
</head>

<body>
    <ul>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
    </ul>
</body>

</html>